As a battery operated industrial truck equipped with a load handling hydraulic device having an electric motor for driving a hydraulic motor, a forklift truck is known for example, in which a hydraulic pump is allowed to function as a hydraulic motor driven by working oil returning from a lift cylinder when a load is lowered and an electric motor connected to the pump is allowed to function as an electric generator to charge the battery to recover the potential energy of the load.
There has been disclosed energy recovering systems, for example in Japanese Laid-Open Patent Application No. 2-169499 and in U.S. Pat. No. 5,505,043, in which the hydraulic pump is rotated reversely by the working fluid pushed out from the lift cylinder in the lowering operation to function as an electric generator.
The system disclosed in said JP 2-169499 is composed such that an electromagnetic clutch is provided between the electric motor and the hydraulic pump, power transmission between the hydraulic pump and the motor is disconnected by the electromagnetic clutch when sufficient lowering speed is not obtained due to shortage of working fluid flow owing to insufficient weight of the load and therefore energy recovering is not possible, thereby saving energy.
The lift device disclosed in said U.S. Pat. No. 5,505,043 comprises a pair of a hydraulic pump and a DC motor for supplying oil to the lift cylinder and auxiliary means.
Further, in Japanese Laid-Open Patent Application No. 2003-252588 is disclosed a system in which the electric motor and the hydraulic pump which can be operated also as a hydraulic motor are controlled in rotation speed when recovering the potential energy of the load, flow and pressure of working oil are controlled so that the load is lowered at a predetermined speed, thereby eliminating energy loss due to pressure loss and heat generation.
However, with the systems disclosed in said JP 2-169499 and U.S. Pat. No. 5,505,043, potential energy of the load can be effectively utilized, but there is a problem that supplying pressurized oil to other actuators to actuate other devices is not possible by using the same hydraulic pump connected to the electric motor while operation of supplying working oil to the lift cylinder or recovering potential energy of the load is performed.
Further, the lift device disclosed in said U.S. Pat. No. 5,505,043 has disadvantages that a plurality of hydraulic pumps and a plurality of electric motors are needed, construction becomes complicated, and weight is increased.
The system disclosed in said JP 2003-252588 has a disadvantage that oil pressure pulsation occurs while recovering the potential energy of the load, which deteriorates operability of the recovering system and reduces energy recovering efficiency.